Update: well, well. The melody does not become swiftly unrecognizable, but is actually pretty clear the whole way. I guess the entire idea of the passage is to introduce lots of folk melodies in different keys, which would have been obvious to people who actually sang them. Silly me! How obvious.
Original post: A friend of mine showed me the delightful passage of Biber’s Battalia that imitates the sounds of the drunk singing in “Die liederliche gselschafft von allerley Humor”. I don’t know if this is obvious, but I may have found “Kraut und Ruben” hidden fairly early on there, becoming subsequently (and swiftly) unrecognizable in the mess. Maybe there are all sorts of German folk melodies in there, but I recognized this one because of its appearance in Bach’s Goldberg Variations.
Highlighted in red is pretty distinctly (without the final passing tone between the E and C natural) the following German folk tune (“Cabbage and turnips are driving me away”):
You can find a fantastic visual aid to understanding Bach’s 30th Variation here at Bach-cantatas.com.